Switching element method of driving switching element rewritable logic integrated circuit and memory

ABSTRACT

A switching element has an ion conductor capable of conducting metal ions for use in an electrochemical reaction therein, a first electrode and a second electrode which are disposed in contact with said ion conductor and spaced a predetermined distance from each other, and a third electrode disposed in contact with the ion conductor. When a voltage for causing the switching element to transit to an on state is applied to the third electrode, metal is precipitated between the first electrode and the second electrode by metal ions, electrically interconnecting the first electrode and the second electrode. When a voltage for causing the switching element to transit to an off state is applied to the third electrode, the precipitated metal is dissolved to electrically disconnect the first electrode and the second electrode from each other.

TECHNICAL FIELD

The present invention relates to a switching element utilizing anelectrochemical reaction, a method of driving the switching element, anFPL (Field Programmable Logic: a rewritable logic integrated circuit)circuit and a memory element which employ the switching element.

BACKGROUND ART

Of memory integrated circuits, switching elements having a nonvolatilefunction capable of being kept on or off even when their power supply isswitched off include an antifuse element as a first conventional exampleand an EEPROM (Electrical Erasable Programmable Read Only Memory) as asecond conventional example.

Switching elements for performing a nonvolatile function based on anelectrochemical reaction include a timer (or an electrochemical timeswitching device) as a third conventional example and a PCRAM(Programmable Conductor Random Access Memory) as a fourth conventionalexample.

The antifuse element as the first conventional example is a switchingelement having two states, i.e., electrically on and off states, and canirreversibly transit from the off state to the on state according to anelectrical or physical process. The antifuse element as the firstconventional example is disclosed in U.S. Pat. No. 5,070,384 and U.S.Pat. No. 5,387,812. The antifuse element is usually formed between twointerconnects. When a high voltage is selectively applied between theinterconnects, the antifuse element is programmed (transiting from theoff state to the on state), electrically interconnecting theinterconnects. Even after the voltage is turned off, the antifuseelement remains in the on state.

The EEPROM as the second conventional example as disclosed in U.S. Pat.No. 4,203,158 has a floating gate electrode disposed between the controlgate electrode and channel layer of a transistor. When the floating gateelectrode stores electric charges, i.e., when it is charged, or when thefloating gate electrode discharges electric charges, i.e., when it isdischarged, the threshold voltage of the transistor changes. Thefloating gate electrode is charged or discharged by injecting electronsinto the floating gate electrode or discharging electrons from thefloating gate electrode in the form of a tunnel current flowing throughan oxide film. Since the floating gate electrode is surrounded by aninsulating film, the electric charges stored therein are not lost afterthe EEPROM is switched off. Therefore, the EEPROM has a nonvolatilecapability.

In recent years, antifuse elements and EEPROMs are used in FPL circuitswhich are integrated circuits whose hardware configuration can bechanged for each application. One example of an FPL circuit is disclosedin Japanese laid-open patent publication No. 8-78532. The disclosed FPLcircuit has a plurality of logic circuit blocks, interconnectsinterconnecting the logic circuit blocks, and antifuse elements forchanging the connection of the interconnects. The antifuse elements areused as programming elements. The antifuse elements selected by the userconnect interconnects. Therefore, the FPL circuit provides a differenthardware configuration for a selection of antifuse elements to connectinterconnects. FPL circuits offer many advantages in that they are moreversatile than ASICs (Application Specific Integrated Circuits) and canbe manufactured inexpensively in a short turnaround time, and arefinding a rapidly growing market.

The timer as the third conventional example has a closed loop made up ofa DC power supply, a load, and first and second inner electrodes. Partof the first and second inner electrodes is immersed in an electrolyticsolution and electroplated, and one of the first and second innerelectrodes is cut off, setting a time for the timer. The timer as thethird conventional example is disclosed in Japanese laid-open utilitymodel publication No. 2-91133.

The electronic element as the fourth conventional example, as disclosedin U.S. Pat. No. 6,348,365, is a PCRAM utilizing silvergermanide/selenide which is a silver ion conductive ion conductivematerial (the term “ion conductive material” has the same meaning as“ion conductor” used in the present specification) as a material forconducting ions.

FIG. 1 of the accompanying drawings is a schematic cross-sectional viewshowing a structure of the PCRAM disclosed in U.S. Pat. No. 6,348,365.As shown in FIG. 1, insulating material 81, conductive material 82, anddielectric material 83 are successively disposed on semiconductorsubstrate 87, and dielectric material 83 partly has a recessed structure(grooved structure). Ion conductive material 86 and metal material 84are disposed in the recessed structure, and electrode 85 is disposed onmetal material 84 and dielectric material 83. When a voltage is appliedbetween electrode 85 and conductive material 82, a current path referredto as a dendrite grows on the surface of ion conductive material 86,thus electrically connecting electrode 85 and conductive material 82 toeach other. When a reverse voltage is applied, the dendrite disappears,electrically isolating electrode 85 and conductive material 82 from eachother.

The antifuse element as the first conventional example is a switchingelement used mainly in FPL circuits. Since the on-resistance, which isthe resistance of the antifuse element when it is in the on state, issmall (about 50Ω), the antifuse element has a small signal delay time.However, the antifuse element is problematic in that it is notreprogrammable. Consequently, when the FPL circuit is programmed, itfails to meet demands for debugging the program and changing programswhile it is in operation.

While the EEPROM as the second conventional example is reprogrammable,the level of integration thereof is low at present and the on-resistancethereof is of a large value of several kΩ because it is limited by theresistance of the MOS (Metal Oxide Semiconductor) transistor. Though theEEPROM is widely used as nonvolatile memory, the level of integrationthereof is limited by the thickness of the insulating film, making itdifficult to further integrate the EEPROM. In addition, when the EEPROMis used in an FPL circuit, it tends to cause a signal delay due to thelarge on-resistance.

The timer as the third conventional example is a device for measuringtime until the electrode is dissolved by an electroplating process whichis based on an electrochemical reaction. The timer cannot operate as aswitching element for switching between on and off states.

The electronic element as the fourth conventional example is basically atwo-terminal switch utilizing an electrochemical reaction. Thetransition of the two-terminal switch between on and off states iscontrolled by a voltage that is applied between the two terminals of theswitch. When the transition occurs between the on and off states, acurrent flows through the switch, and the switch consumes a large amountof electric power. The switch requires thick interconnects that canwithstand the current that is needed to cause the transition between theon and off states, and also requires a transistor having large drivepower. Even though the switch itself can be integrated, it is difficultto integrate the interconnects and peripheral circuits.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a switching elementwhich can be highly integrated, can be kept in either one of an on stateand an off state even when the switching element is switched off, has alow resistance when it is in the on state, and can be reprogrammed intoeither one of the on and off states, a method of driving the switchingelement, and an FPL circuit and a memory element which employ theswitching element.

A switching element according to the present invention has an ionconductor capable of conducting metal ions therein, a first electrodeand a second electrode which are disposed in contact with the ionconductor, and a third electrode disposed in contact with the ionconductor and including the metal ions, wherein an interelectrodedistance L1 between the first electrode and the second electrode, aninterelectrode distance L2 between the first electrode and the thirdelectrode, and an interelectrode distance L3 between the secondelectrode and the third electrode satisfy the condition according to theexpression:L1<L2×2 and L1<L3×2.

According to the present invention, metal can be precipitated betweenthe first electrode and the second electrode, and the precipitated metalcan be dissolved by controlling a voltage applied to the thirdelectrode. The switching element can switch between a state in which thefirst electrode and the second electrode are electrically interconnectedand a state in which the first electrode and the second electrode arenot electrically interconnected.

In the switching element according to the present invention, theinterelectrode distance between the first electrode and the secondelectrode may be 0.5 μm or less. If the interelectrode distance betweenthe first electrode and the second electrode is 0.5 μm or less, then theswitching element according to the present invention can be incorporatedinto various integrated circuits.

The switching element according to the present invention may be disposedon a substrate covered with an insulating film or an insulatingsubstrate. According to an aspect of this arrangement, the firstelectrode and the second electrode may be disposed on the substrate sothat they are separated from each other, and the interelectrode distancebetween the first electrode and the second electrode may be 0.5 μm orless, the ion conductor may be disposed to cover the first electrode andthe second electrode, and the third electrode may be disposed on the ionconductor. According to another aspect, the third electrode may bedisposed on the substrate, the ion conductor may be disposed on thethird electrode, the first electrode and the second electrode may bedisposed on the ion conductor in spaced-apart relation to each other,and the interelectrode distance between the first electrode and thesecond electrode may be 0.5 μm or less. According to still anotheraspect, the first electrode may be disposed on the substrate, the ionconductor may be disposed on the first electrode, the second electrodeand the third electrode may be disposed on the ion conductor, and theinterelectrode distance between the first electrode and the secondelectrode may be equal to or greater than the film thickness of the ionconductor. With either one of these aspects, an integrated circuitcombined with a semiconductor element can be easily formed.

In the switching element according to the present invention, anelectrical characteristic between the first electrode and the secondelectrode may be controlled by applying a voltage to the thirdelectrode. The electrical characteristic may represent electricconductivity.

In the switching element according to the present invention, the firstelectrode and the second electrode may be electrically interconnected tobring the switching element into an on state by applying a voltage,which is positive with respect to at least one of the first electrodeand the second electrode, to the third electrode, and the firstelectrode and the second electrode may be insulated from each other tobring the switching element into an off state by applying a voltage,which is negative with respect to at least one of the first electrodeand the second electrode, to the third electrode.

In the switching element according to the present invention, the secondelectrode may include a metal dissolvable into the ion conductor basedon an electrochemical reaction. In this case, the first electrode andthe second electrode may be electrically interconnected to bring theswitching element into an on state by either applying a voltage, whichis positive with respect to the first electrode, to the secondelectrode, or applying a voltage, which is positive with respect to atleast one of the first electrode and the second electrode, to the thirdelectrode, and the first electrode and the second electrode may beinsulated from each other to bring the switching element into an offstate by either applying a voltage, which is negative with respect tothe first electrode, to the second electrode, or applying a voltage,which is negative with respect to at least one of the first electrodeand the second electrode, to the third electrode.

In the switching element according to the present invention, at leastone of the first electrode, the second electrode, and the thirdelectrode may have a pointed portion on a surface thereof held incontact with the ion conductor.

In the switching element according to the present invention, the ionconductor may comprise either a calcogenide material including anelement belonging to the group 6B of the periodic table, metal ionicglass, or metal ionic amorphous semiconductor.

In the switching element according to the present invention, the ionconductor and the third electrode may be made of either copper sulfideand copper, respectively, or silver sulfide and silver, respectively,and portions of the first electrode and the second electrode which areheld in contact with the ion conductor may be made of either a metalsuch as platinum, aluminum, gold, titanium, tungsten, vanadium, niobium,tantalum, chromium, or molybdenum, a nitride of the metal, a silicide ofthe metal, or a combination thereof.

A method of driving a switching element according to the presentinvention has the step of controlling the electrical characteristicbased on the voltage applied to the third electrode and/or a time inwhich the voltage is applied to the third electrode.

A method of driving a switching element according to the presentinvention has the steps of selectively bringing the switching elementinto the on state and the off state depending on the polarity of thevoltage applied to the third electrode, and holding the switchingelement in either the on state or the off state by stopping applicationof the voltage to the third electrode. When the switching element iscaused to transit between the on state and the off state, theconductivity between the first electrode and the second electrode may bemeasured, and the voltage applied to the third electrode may becontrolled based on a change in the conductivity.

A rewritable logic integrated circuit according to the present inventionincorporates a switching element according to the present invention as aprogramming switch.

A memory device according to the present invention has memory cells eachcomprising a switching element according to the present invention andeither a MOS transistor or a diode. The memory cell may comprise a MOStransistor, the switching element may have the second electrodeconnected to the drain electrode of the MOS transistor, the firstelectrode to a first bit line, and the third electrode to a first wordline, and the MOS transistor may have a source electrode connected to asecond bit line different from the first bit line and a gate electrodeconnected to a second word line different from the first word line.

A switching element according to the present invention is a switchingelement utilizing an electrochemical reaction, and has an ion conductorcapable of conducting metal ions for use in an electrochemical reactiontherein, a first electrode and a second electrode which are disposed incontact with the ion conductor and spaced a predetermined distance fromeach other, and a third electrode disposed in contact with the ionconductor, for precipitating a metal between the first electrode and thesecond electrode due to metal ions when a voltage for causing theswitching element to transit to an on state is applied to the thirdelectrode, and dissolving the precipitated metal to electricallydisconnect the first electrode and the second electrode from each otherwhen a voltage for causing the switching element to transit to an offstate is applied to the third electrode.

According to the present invention, when the voltage for causing theswitching element to transit to the on state is applied to the thirdelectrode, metal ions in the ion conductor are attracted to the firstelectrode and the second electrode by an electrochemical reaction,precipitating metal on the surfaces of these electrodes to electricallyinterconnect the first electrode and the second electrode with the metalprecipitated between these electrodes. When the voltage for causing theswitching element to transit to the off state is applied to the thirdelectrode, the metal precipitated between the first electrode and thesecond electrode is dissolved as metal ions into the ion conductor,electrically disconnecting the first electrode and the second electrodefrom each other. In the on state, since the first electrode and thesecond electrode are interconnected by the metal, the resistance betweenthe first electrode and the second electrode is made smaller.

In the switching element according to the present invention, when theapplication of the voltage to the third electrode is stopped after theswitching element has been brought into the on state or the off state,the switching element is held in that state.

According to the present invention, even when no voltage is applied tothe third electrode after the switching element has been brought intothe on state, the first electrode and the second electrode remainelectrically interconnected by the precipitated metal. Even when novoltage is applied to the third electrode after the switching elementhas been brought into the off state, the first electrode and the secondelectrode remain electrically disconnected from each other. Therefore,the switching element is rendered nonvolatile, holding information inthe on state or the off state.

A switching element according to the present invention is a switchingelement utilizing an electrochemical reaction, and has an ion conductorcapable of conducting metal ions for use in an electrochemical reactiontherein, a first electrode disposed in contact with the ion conductor, asecond electrode disposed in contact with the ion conductor and spaced apredetermined distance from the first electrode, for precipitating ametal due to metal ions to electrically connect the second electrode tothe first electrode when a voltage for causing the switching element totransit to an on state is applied to the second electrode, anddissolving the precipitated metal to electrically disconnect the secondelectrode from the first electrode when a voltage for causing theswitching element to transit to an off state is applied to the secondelectrode, and a third electrode disposed in contact with the ionconductor for increasing a current flowing between the first electrodeand the creasing a current flowing between the first electrode and thesecond electrode when a voltage, which is positive with respect to thefirst electrode, is applied to the third electrode, and reducing thecurrent when a voltage, which is negative with respect to the firstelectrode, is applied to the third electrode.

According to the present invention, when the voltage for causing theswitching element to transit to the on state is applied to the secondelectrode, metal ions in the ion conductor are attracted to the firstelectrode by an electrochemical reaction, precipitating a metal on thesurface of the first electrode to electrically interconnect the firstelectrode and the second electrode with the metal precipitated betweenthese electrodes. When a voltage, which is positive with respect to thefirst electrode, is applied to the third electrode, the amount of metalprecipitated between the first electrode and the second electrodeincreases, increasing the current flowing therebetween. When a voltage,which is negative with respect to the first electrode, is applied to thethird electrode after the switching element has been brought into the onstate, the amount of metal precipitated between the first electrode andthe second electrode decreases, reducing the current flowingtherebetween. When the voltage for causing the switching element totransit to the off state is applied to the second electrode, the metalprecipitated between the first electrode and the second electrode isdissolved as metal ions into the ion conductor, electricallydisconnecting the first electrode and the second electrode from eachother. Therefore, the on state and the off state can be controlled byapplying the voltage to the second electrode, and the magnitude of thecurrent can be controlled by applying the voltage to the thirdelectrode.

A switching element according to the present invention is a switchingelement utilizing an electrochemical reaction, and has an ion conductorcapable of conducting metal ions for use in an electrochemical reactiontherein, a first electrode disposed in contact with the ion conductor, asecond electrode disposed in contact with the ion conductor and spaced apredetermined distance from the first electrode, for precipitating ametal due to metal ions when a predetermined voltage is applied to thesecond electrode for a predetermined time, and a third electrodedisposed in contact with the ion conductor for precipitating a metal dueto metal ions to electrically interconnect the first electrode and thesecond electrode when a voltage for causing the switching element totransit to an on state is applied to the third electrode after thepredetermined voltage has been applied to the second electrode for thepredetermined time.

According to the present invention, when the voltage for causing theswitching element to transit to the on state is applied to the thirdelectrode after the voltage has been applied to the second electrodebefore the first electrode and the second electrode are interconnectedby the metal precipitated by an electrochemical reaction, the firstelectrode and the second electrode are electrically connected to eachother. Therefore, when the first electrode and the second electrode areelectrically connected to each other, an excessive current is preventedfrom flowing, and the electric power consumed by the switching elementis reduced.

In the switching element according to the present invention, the thirdelectrode may include a material for supplying metal ions to the ionconductor, and portions of the first electrode and the second electrodewhich are held in contact with the ion conductor may be made of amaterial which does not react with the ion conductor.

According to the present invention, since metal ions are supplied fromthe third electrode to the ion conductor by an electrochemical reaction,the ion conductivity is increased, increasing the speed at which theswitching element transits between the on state and the off state.

In the switching element according to the present invention, the thirdelectrode and the second electrode may include a material for supplyingmetal ions to the ion conductor, and a portion of the first electrodewhich is held in contact with the ion conductor may be made of amaterial which does not react with the ion conductor.

According to the present invention, since metal ions are supplied fromthe third electrode and the second electrode to the ion conductor by anelectrochemical reaction, the ion conductivity is increased, increasingthe speed at which the switching element transits between the on stateand the off state.

In the switching element according to the present invention, the firstelectrode and the second electrode may be formed in one plane parallelto a planar pattern of the third electrode, at least one of the firstelectrode and the second electrode may have a planar pattern having apointed portion, and the shortest distance between the first electrodeand the second electrode may be represented by the distance from thepointed portion of one of the electrodes to the other electrode.

According to the present invention, since the shortest distance betweenthe electrodes is equal to the distance from the pointed portion of oneof the electrodes to the other electrode, the electrode with the pointedportion can be electrically connected to the other electrode when copperis precipitated at least in the vicinity of the pointed portion.Therefore, excessive copper does not need to be precipitated, allowingthe switching element to transit from the off state to the on state atan increased speed. The switching element is also allowed to transitfrom the on state to the off state at an increased speed because themetal in the vicinity of the pointed portion may be dissolved forbringing the switching element into the off state.

The rewritable logic integrated circuit according to the presentinvention which will achieve the above object incorporates either one ofthe above switching elements according to the present invention as aprogramming switch. According to the present invention, a logic circuitcan be freely established by causing the switching element used as theprogramming element to be in the on state or the off state.

The memory device according to the present invention which will achievethe above object has either one of the above switching elementsaccording to the present invention and a transistor for readinginformation indicative of whether the switching element is in the onstate or the off state. According to the present invention, after theswitching element has been brought into the on state or the off statebased on an electrochemical reaction, the switching element is held inthat state even if no voltage is applied to the third electrode and thesecond electrode. The memory device can thus be used as a nonvolatilememory.

According to the present invention, therefore, there is provided aswitching element which can be set to the on state or the off statedesirably by applying a predetermined voltage to at least one of thethird electrode and the second electrode, and which is nonvolatile andhas smaller resistance when it is in the on state. Furthermore, inasmuchas the switching element according to the present invention is of asimple and minute structure, it can be manufactured in a much smallerdesign than heretofore.

If the switching element according to the present invention isincorporated in an FPL circuit, then the FPL circuit is reprogrammableand can operate at a high speed.

If the switching element according to the present invention is used asan information storage means in a memory device, then the memory deviceis available as nonvolatile memory having high writing and readingrates. Inasmuch as the switching element according to the presentinvention is of a simple and minute structure, the memory device can bemanufactured as a highly integrated, high-speed memory device.

With a manufacturing process according to the present invention, theswitching element can be consistently and accurately manufactured, usingthe conventional technology for manufacturing semiconductor integratedcircuits. Therefore, the switching element, and the FPL circuit and thememory device incorporating the switching element can be produced at alow cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an electronic element as afourth conventional example;

FIG. 2 is a cross-sectional view showing the structure of a switchingelement according to the present invention;

FIG. 3A is a graph showing electrical characteristics of the switchingelement according to the present invention;

FIG. 3B is a graph showing electrical characteristics of the switchingelement according to the present invention;

FIG. 4 is a view illustrative of an electrochemical reaction of theswitching element according to the present invention;

FIG. 5 is a cross-sectional view of a structure of a switching elementaccording to the present invention;

FIG. 6A is a plan view showing an example of a planar pattern of asource electrode and a drain electrode;

FIG. 6B is a plan view showing another example of a planar pattern of asource electrode and a drain electrode;

FIG. 7 is a flowchart of a sequence for the feedback control of a gatevoltage;

FIG. 8A is a cross-sectional view of another structure of the switchingelement according to the first embodiment;

FIG. 8B is a cross-sectional view of still another structure of theswitching element according to the first embodiment;

FIG. 8C is a cross-sectional view of yet another structure of theswitching element according to the first embodiment;

FIG. 9 is a graph showing electrical characteristics of the switchingelement according to the first embodiment in which an ion conductor ismade of copper sulfide produced by anodic polarization;

FIG. 10 is a graph showing electrical characteristics of the switchingelement according to the first embodiment in which an ion conductor ismade of copper sulfide produced by laser ablation;

FIG. 11 is a cross-sectional view of a structure of a switching elementaccording to the second embodiment of the present invention;

FIG. 12 is a cross-sectional view showing a structure of a switchingelement according to the present invention as it is applied to an FPLcircuit; and

FIG. 13 is a circuit diagram of a memory device comprising switchingelements according to the present invention and MOS transistors.

BEST MODE FOR CARRYING OUT THE INVENTION

A switching element according to the present invention is characterizedas follows: A voltage applied to a third electrode is controlled toprecipitate a metal between a first electrode and a second electrode toelectrically interconnect the first electrode and the second electrode,i.e., to achieve an on state. The metal precipitated between the firstelectrode and the second electrode is dissolved to disconnect the firstelectrode and the second electrode, i.e., to achieve an off state. Eachof these states is kept even if the voltage is no longer applied to thethird electrode.

Arrangements of the present invention will be described below.

In the following description and FIGS. 2, 3A, 3B, 5, 6A, 6B, 8A, 8B, 8C,9, 10, 11, and 12, the first electrode corresponds to a sourceelectrode, the second electrode to a drain electrode, and the thirdelectrode to a gate electrode.

FIG. 2 is a cross-sectional view showing a structure of a switchingelement according to the present invention.

As shown in FIG. 2, the switching element according to the presentinvention has source electrode 1 and drain electrode 2 disposed onsubstrate 5 comprising a silicon substrate covered with a silicon oxidefilm as an insulating film and spaced a predetermined distance from eachother, ion conductor 4 disposed in contact with source electrode 1 anddrain electrode 2 and including metal ions for an electrochemicalreaction, and gate electrode 3 disposed on ion conductor 4. Gateelectrode 3 serves to control conductivity between source electrode 1and drain electrode 2 depending on the magnitude of the voltage appliedto gate electrode 3. Source electrode 1, drain electrode 2, and gateelectrode 3 are electrically insulated from each other.

Gate electrode 3 includes a material for supplying metal ions to ionconductor 4 based on an electrochemical reaction. The portions of sourceelectrode 1 and drain electrode 2 which are held in contact with ionconductor 4 are made of a material which does not electrochemicallyreact with ion conductor 4. Therefore, source electrode 1 and drainelectrode 2 do not supply metal ions to ion conductor 4.

Operation of the switching element thus constructed will be describedbelow.

When a voltage, which is positive with respect to source electrode 1 anddrain electrode 2, is applied to gate electrode 3, a metal isprecipitated on source electrode 1 and drain electrode 2, which arepositioned closely to each other, due to a reduction reaction of metalions. The metal that is precipitated in an interelectrode gap 6, acrosswhich source electrode 1 and drain electrode 2 are spaced from eachother by the predetermined distance, electrically interconnects sourceelectrode 1 and drain electrode 2, whereupon the switching elementtransits to an on state. When a voltage, which is negative with respectto source electrode 1 and drain electrode 2, is applied to gateelectrode 3, the metal precipitated in interelectrode gap 6 is oxidizedinto metal ions, which are dissolved into ion conductor 4. The metal isnow removed from interelectrode gap 6, causing the switching element totransit to an off state. Interelectrode gap 6 represents the shortestdistance between source electrode 1 and drain electrode 2.

The on and off states of the switching element are kept even if thevoltages are no longer applied to gate electrode 3. After the on stateis reached, the metal is precipitated or dissolved depending on the timefor which the voltage is applied to gate electrode 3 and the appliedvoltage, allowing the voltage applied to gate electrode 3 to control theconductivity between source electrode 1 and drain electrode 2.

If the interelectrode distance between source electrode 1 and drainelectrode 2 is represented by L1, the interelectrode distance betweensource electrode 1 and gate electrode 3 by L2, and the interelectrodedistance between drain electrode 2 and gate electrode 3 by L3, thensource electrode 1 drain electrode 2, and gate electrode 3 can bepositioned to satisfy the condition according to the followingexpression (1):L1<L2×2 and L1<L3×2  (1)

With the switching element constructed to satisfy the conditionaccording to the expression (1), when a metal is grown from both sourceelectrode 1 and drain electrode 2, source electrode and gate electrodeor drain electrode and gate electrode are essentially prevented frombeing electrically interconnected before source electrode and drainelectrode are electrically interconnected by the grown metal.

Source electrode 1, drain electrode 2, and gate electrode 3 may bepositioned to satisfy the condition according to the followingexpression (2):L1<L2×1 and L<L3×1  (2)

With the switching element constructed to satisfy the conditionaccording to the expression (2), when a metal is grown from either oneof source electrode and drain electrode, source electrode and gateelectrode or drain electrode and gate electrode are essentiallyprevented from being electrically interconnected before source electrodeand drain electrode are electrically interconnected by the grown metal.If the electrical conductivity between source electrode 1 and drainelectrode 2 is to be controlled by the voltage applied to gate electrode3, the arrangement which meets the condition according to the expression(2) makes it possible to increase a variable margin of the voltageapplied to gate electrode 3.

Furthermore, source electrode 1, drain electrode 2, and gate electrode 3may be positioned to satisfy the condition according to the followingexpression (3):L1<L2×½ and L1<L3×½  (3)

With the switching element constructed to satisfy the conditionaccording to the expression (3), source electrode and gate electrode ordrain electrode and gate electrode are reliably prevented from beingelectrically interconnected by metal grown from source electrode and/ordrain electrode.

The smaller the interelectrode distance L1 between source electrode 1and drain electrode 2, the lower the voltage applied to gate electrode3, allowing the switching element to consume smaller electric power andalso allowing the switching element to switch between the on state andthe off state faster. If the interelectrode distance L1 is 0.5 μm orsmaller, then the switching element according to the present inventioncan be incorporated into various integrated circuits. However, if theinterelectrode distance L1 is too small, leakage current flowing when avoltage is applied between source electrode and drain electrodeincreases. The interelectrode distance L1 may be set such that theleakage current will be 1/10 or less of the current flowing betweensource electrode and drain electrode.

Electrical characteristics of the switching element shown in FIG. 2 willbe described below.

FIGS. 3A and 3B are graphs showing electrical characteristics of theswitching element. In FIGS. 3A and 3B, the horizontal axis representsthe gate voltage which is the voltage applied to gate electrode 3 of theswitching element shown in FIG. 2, and the vertical axis the draincurrent which is current flowing between source electrode 1 and drainelectrode 2.

In the switching element used to make the measurement shown in FIG. 3A,ion conductor 4 comprises an aqueous solution of copper sulfide, sourceelectrode 1 and drain electrode 2 are made of platinum (Pt) which is notdissolvable into ion conductor 4, and gate electrode 3 is made of copper(Cu) which is capable of electrochemically reacting with ion conductor4.

As shown in FIG. 3A, when a constant voltage is applied between drainelectrode 2 and source electrode 1 and a potential difference betweengate electrode 3 and source electrode 1 is repeatedly varied, theconductivity between drain electrode 2 and source electrode 1 exhibitshysteresis. The hysteretic conductivity will be described in detailbelow.

In an initial state with no voltage applied, the switching element is inthe off state and almost no drain current flows. When the gate voltageapplied to gate electrode 3 is changed positively from 0 V to +0.3 V inthe off state, a drain current of about 1.2 mA flows, causing theswitching element to transit to the on state. When the gate voltage ischanged negatively to −0.16 V in the on state, almost no drain currentflows, causing the switching element to transit to the off state. Thus,when the gate voltage is in the range from −0.16 V to +0.3 V, no switchtransition occurs, and the switching element is stably held in the onstate or the off state. It is possible to cause the switching element totransit between the on state and the off state as many times as desiredby repeatedly varying the gate electrode.

The reasons why the switching element transits between the on state andthe off state as shown in FIG. 3A will be described below.

FIG. 4 is a view illustrative of the precipitation and dissolution ofcopper due to an electrochemical reaction.

As shown in FIG. 4, a gold electrode and a copper electrode are immersedin an ion conductor in the form of a mixed solution of copper sulfateand sulfuric acid, and a voltage is applied from a voltage source to thecopper electrode as a positive electrode and the gold electrode as anegative electrode. Since the mixed solution is a copper platingsolution, copper of the copper electrode is dissolved as copper ionsinto the ion conductor, and copper is precipitated on the goldelectrode. Copper is thus precipitated and dissolved by such anelectrochemical reaction.

The switching element according to the present invention utilizes theelectrochemical reaction shown in FIG. 4. The copper electrode shown inFIG. 4 corresponds to gate electrode 3 shown in FIG. 2, and the goldelectrode shown in FIG. 4 corresponds to source electrode 1 and drainelectrode 2 shown in FIG. 2.

The electrochemical reaction shown in FIG. 4 will be described withrespect to the switching element shown in FIG. 2.

When copper is precipitated on the surfaces of source electrode 1 anddrain electrode 2 by the above electrochemical reaction, interelectrodegap 6 is filled with copper, electrically interconnecting sourceelectrode 1 and drain electrode 2 to cause the switching element totransit to the on state. After the switching element has transited tothe on state shown in FIG. 3A, when the applied gate voltage is madegreater than +0.3 V, the drain current increases with the gateelectrode. This means that conductivity between source electrode 1 anddrain electrode 2 increases as the precipitated amount of copperincreases.

When the copper precipitated in interelectrode gap 6 is dissolved intoion conductor 4 by the electrochemical reaction, the copper is removedfrom interelectrode gap 6, electrically disconnecting source electrode 1and drain electrode 2, causing the switching element to transit to theoff state.

The speed of transition between the on state and the off state will bedescribed below.

When copper ions dissolved from gate electrode 3 travel to the surfaceof source electrode 1 or drain electrode 2 and are coupled to electrons,copper is precipitated, electrically interconnecting source electrode 1and drain electrode 2. When the copper that fills interelectrode gap 6between source electrode 1 and drain electrode 2 is dissolved, sourceelectrode 1 and drain electrode 2 are electrically disconnected fromeach other. The speed of transition between the on state and the offstate is thus determined by the speed at which the metal ions travel inion conductor 4 and the rate of the electrochemical reaction. The speedat which the metal ions travel in ion conductor 4 depends upon the ionconductivity and the gate voltage.

The switching element which exhibits the electrical characteristicsshown in FIG. 3B will be described below.

In the switching element used to make the measurement shown in FIG. 3B,ion conductor 4 comprises an aqueous solution of copper sulfide, sourceelectrode 1 is made of platinum (Pt), and gate electrode 3 and drainelectrode 2 are made of copper (Cu) which is capable ofelectrochemically reacting with ion conductor 4.

As shown in FIG. 3B, when a constant voltage is applied between drainelectrode 2 and source electrode 1 and a potential difference betweengate electrode 3 and source electrode 1 is repeatedly varied, theconductivity between drain electrode 2 and source electrode 1 exhibitshysteresis.

The hysteretic conductivity will be described in detail below. In aninitial state with no voltage applied, the switching element is in theoff state and almost no drain current flows. When the gate voltageapplied to gate electrode 3 is changed positively from 0 V to +0.75 V inthe off state, a drain current of about 2 mA flows, causing theswitching element to transit to the on state. When the gate voltage ischanged negatively to −0.4 V in the on state, almost no drain currentflows, causing the switching element to transit to the off state. Thus,when the gate voltage is in the range from −0.4 V to +0.75 V, no switchtransition occurs, and the switching element is stably held in the onstate or the off state. It is possible to cause the switching element totransit between the on state and the off state as many times as desiredby repeatedly varying the gate electrode.

In the arrangements of the switching element used to make themeasurements shown in FIGS. 3A and 3B, since drain electrode is made ofcopper which is capable of electrochemically reacting with ion conductor4, the switching element may be caused to transit to the on state or theoff state by applying a voltage between drain electrode and sourceelectrode.

In the above example, ion conductor 4 comprises an electrolytic solutionin the form of a mixed aqueous solution of copper sulfate and sulfuricacid. However, ion conductor 4 in other forms also produces the sameeffect as described above. Ion conductors are roughly classified intotwo types, i.e., a liquid and a solid. The liquid ion conductor is anelectrolytic solution described above, and the solid ion conductor is asolid electrolyte wherein metal ions can freely move as in a solution.If the switching element is incorporated into an integrated circuit,then the solid ion conductor is suitable for use in the switchingelement. Silver ions and copper ions, in particular, exhibit ionconductivity in an appropriate solid electrolyte, e.g., silver sulfideor copper sulfide. The inventors have found that like copper ions in amixed aqueous solution of copper sulfate and sulfuric acid, silver ionsin silver sulfide and copper ions in copper sulfide exhibit a switchingphenomenon based on transition between the on state and the off state.Known materials in which silver ions and copper ions travel includemetal ionic glass and metal ionic amorphous semiconductor, other thancalcogenides including elements which belong to group 6B of the periodictable.

Elements based on the above principles of operation have not heretoforebeen known in the art, and the inventors have devised and verified forthe first time the mechanism of such elements.

1st Embodiment

Of switching elements according to the present invention, a switchingelement employing a solid electrolyte as an ion conductor will bedescribed below.

FIG. 5 is a cross-sectional view of the structure of a switching elementaccording to the present invention. As shown in FIG. 5, switchingelement 10 has gate electrode 13 disposed on substrate 15 covered with asilicon oxide film as an insulating film, ion conductor 14 disposed ongate electrode 13, and source electrode 11 and drain electrode 12disposed on ion conductor 14. Source electrode 11 and drain electrode 12are disposed within one plane, with a gap of 100 nm or less definedtherebetween. Source electrode 11, drain electrode 12, and gateelectrode 13 are electrically insulated from each other.

Gate electrode 13 includes a material for supplying metal ions to ionconductor 14 based on an electrochemical reaction. Ion conductor 14should preferably comprise a solid electrolyte with as small electronconductivity as possible because the greater the electron conductivity,the greater the leakage current flowing when switching element 10 is inthe off state. The portions of source electrode 11 and drain electrode12 which are held in contact with ion conductor 14 are made of amaterial which does not electrochemically react with ion conductor 14.Therefore, source electrode 11 and drain electrode 12 do not supplymetal ions to ion conductor 14 though they are held in contact with ionconductor 14.

Materials which do not react with ion conductor 14 include metals suchas platinum, aluminum, gold, titanium, tungsten, vanadium, niobium,tantalum, chromium, molybdenum, etc. Materials which are hardlychemically reactive and ionizable may be nitrides of the above metals orsilicon compounds (silicides) such as suicides of the above metals. Theportions of source electrode 11 and drain electrode 12 which are held incontact with ion conductor 14 are not required to be made of a commonmaterial, but may each be made of either one of the above metals andcompounds.

Planar patterns of source electrode 11 and drain electrode 12 will bedescribed below.

FIGS. 6A and 6B are plan views showing examples of planar patterns ofsource electrode 11 and drain electrode 12.

In FIG. 6A, planar patterns of source electrode 11 and drain electrode12 are rectangular in shape, and the gap between these two electrodes isdefined between two parallel sides thereof.

In FIG. 6B, planar patterns of source electrode 11 and drain electrode12 are polygonal in shape, and the gap between these two electrodes,which represents the shortest distance therebetween, is defined betweenrespective vertexes of the patterns. In this case, since sourceelectrode 11 and drain electrode 12 are electrically interconnected bycopper precipitated between the vertexes of the patterns, copper doesnot need to be precipitated excessively, and the switching element isallowed to transit to the on state faster than with the planar patternsof source electrode 11 and drain electrode 12 shown in FIG. 6A. When theprecipitated copper is dissolved to electrically disconnect sourceelectrode 11 and drain electrode 12, the switching element also transitsto the off state faster. Though the gap between source electrode 11 anddrain electrode 12 is illustrated as being defined between the vertexesof their patterns in FIG. 6B, one of the vertexes may be replaced withone side of the pattern. Such a modification is still considered toallow the switching element to transit to the on and off states fasterthan with the planar patterns shown in FIG. 6A. The two electrodes arenot required to be polygonal in shape, but either one of the electrodesmay have a pointed portion like the above vertex.

Operation of the switching element having the above structure will bedescribed below.

Source electrode 11 is grounded, a voltage of +0.1 V is applied to drainelectrode 12, and a positive voltage is applied to gate electrode 13. Adrain current flowing between source electrode 11 and drain electrode 12is observed, causing the switching element to transit to the on state.After the switching element has transited to the on state, when the gatevoltage applied to gate electrode 13 is increased, the drain current isincreased. When a negative voltage is applied to gate electrode 13, thedrain current is reduced, causing the switching element to transit tothe off state.

In order to cause the switching element to transit between the on stateand the off state, the period of time for which the gate voltage isapplied or the applied voltage may be controlled to equalize theresistance between source electrode 11 and drain electrode 12 to adesired target resistance according to the following feedback controlprocess:

FIG. 7 is a flowchart of a feedback control process for controlling thegate voltage. In an experiment, the feedback control process isperformed by a personal computer (hereinafter referred to as PC). The PChas a CPU (Central Processing Unit) for performing predeterminedprocesses according to a program and a memory for storing the program.

As shown in FIG. 7, when a predetermined voltage is applied to gateelectrode 3 (step S101), the PC reads an output current serving as adrain current (step S102), determines the resistance between the twoelectrodes from the value of the applied voltage and the value of theread output current, and compares the determined resistance with apreset target resistance (step S103). The PC stores the value of thevoltage applied to gate electrode 13, the time for which the voltage isapplied to gate electrode 13, and the determined resistance as data inthe memory.

If the determined resistance agrees with the target resistance within apredetermined range in step S103, then the PC ends the application ofvoltage (step S104). If the determined resistance does not fall in thepredetermined range in step S103, then control returns to step S101 inwhich the voltage is applied.

The time required to perform one processing cycle according to theflowchart shown in FIG. 7 is about 100 ms. However, if a dedicatedelectric circuit is employed, then the time required to perform oneprocessing cycle may be reduced to 100 ns or shorter.

By thus feeding back the value of the voltage applied to the gateelectrode, not only the switching element is caused to transit reliablybetween the on state and the off state, but also the on-resistance ofthe switching element and the off resistance thereof, which is theresistance of the switching element when it is in the off state, can bedetermined more accurately.

A process of manufacturing the switching element of the above structurewill be described below.

After a silicon oxide film is formed to a thickness of 300 nm on asemiconductor substrate, a copper film is formed to a thickness of 150nm on the silicon oxide film by vacuum evaporation. Then, a resisthaving a predetermined pattern is formed on the copper film bylithography, and then the area of the copper film which is not coveredwith the resist is removed by ion milling, thus forming gate electrode13. Therefore, ion conductor 14 of copper sulfide is formed to athickness of 100 nm on gate electrode 13 by anodic polarization.

Anodic polarization will be described in detail below. In an aqueoussolution containing 0.025 mol/L of sodium sulfide, gate electrode 13′including copper as a metal to be sulfided is used as an anode and agold electrode is used as a cathode. When a voltage is applied betweengate electrode 13 and the gold electrode, sulfur ions in the aqueoussolution are attracted to the anode, and copper on the surface of gateelectrode 13 turns into copper sulfide due to an electrochemicalreaction. While the progress of sulfidation is being monitored bymeasuring the ion current, ion conductor 14 is formed to a desired filmthickness.

After ion conductor 14 is formed, a titanium film is formed to athickness of 10 nm on ion conductor 14 by sputtering, and then a goldfilm is formed to a thickness of 100 nm on the titanium film by vacuumevaporation. After a resist having a predetermined pattern is formed onthe gold film by lithography, the assembly is dry-etched to form sourceelectrode 11 and drain electrode 12. Thereafter, the resist is removed.When source electrode 11 and drain electrode 12 are formed, aninterelectrode gap having a size of 100 nm or less is definedtherebetween.

While the titanium film is formed by sputtering in the above example, itmay be formed by vacuum evaporation. Lift-off may be used instead of dryetching to form source electrode 11 and drain electrode 12.

Methods other than anodic polarization may be employed to form coppersulfide. For example, copper may be reacted with sulfur at a temperatureof 200° C. or higher in a gas phase to form copper sulfide.Alternatively, a film of copper sulfide may be grown by laser ablation.

According to the present embodiment, the switching element is formed onsubstrate 15 covered with silicon oxide film. However, the switchingelement according to the present embodiment may be formed on aninsulating film which covers MOS transistors and interconnects formed onthe surface of the substrate. This is because even if the switchingelement is formed on the insulating film, it does not essentially affectthe characteristics of the MOS transistors and the interconnects sinceheat treatment is performed at a temperature of 400° C. or lower in theprocess of fabricating the switching element. It is also possible toform another switching element on an insulating film that is formed onthe switching element. Consequently, the switching element according tothe present invention allows a circuit which incorporates the switchingelement to be highly integrated.

The structure of the switching element shown in FIG. 5 is illustrativeonly. Switching elements of other structures are possible withoutdeparting from the scope of the invention. Other structures of switchingelements are shown in FIGS. 8A through 8C.

In FIG. 8A, a switching element is of a recessed structure wherein theion conductor and the gate electrode of the switching element shown inFIG. 5 are embedded in insulating layer 26. An opening is defined abovesubstrate 25 covered with a silicon oxide film, and gate electrode 23and ion conductor 24 are successively disposed in the opening. Sourceelectrode 21 and drain electrode 22 are disposed on ion conductor 24.The gap or distance between source electrode 21 and drain electrode 22is the same as that shown in FIG. 5. With the recessed structure, if aplurality of switching elements are formed, insulating layers 26 thatelectrically insulate adjacent ones of the switching elements have uppersurfaces lying flush with each other, allowing interconnects connectedto source electrode 21 and drain electrode 22 lowing interconnectsconnected to source electrode 21 and drain electrode 22 on insulatinglayers 26 to be planar and hence less liable to be broken.

In FIG. 8B, the gate electrode, the source electrode, and the drainelectrode in the switching element shown in FIG. 8A are verticallyreversed. This arrangement is characterized in that ion conductor 34 isalso disposed in the gap between source electrode 31 and drain electrode32.

In FIG. 8C, source electrode 41 and gate electrode 43 are disposed inone interconnect layer, and drain electrode 42 is disposed in adifferent interconnect layer that is positioned across ion conductor 44from the above interconnect layer. The size of the gap between drainelectrode 42 and source electrode 41 can be established by the filmthickness of ion conductor 44.

FIG. 9 shows electrical characteristics of the switching elementaccording to the present embodiment where copper sulfide is produced byanodic polarization. Source electrode 11 is grounded, a voltage of +0.1V is applied to drain electrode 12, and a positive voltage is applied togate electrode 13. A current flowing between source electrode and drainelectrode is observed, causing the switching element to transit to theon state. When a negative voltage is applied to gate electrode, thecurrent is reduced, causing the switching element to transit to the offstate. Then, after the switching element has transited to the off state,when a positive gate voltage is applied, the current flowing betweensource electrode and drain electrode is increased.

FIG. 10 shows electrical characteristics of the switching elementaccording to the present embodiment where copper sulfide is produced bylaser ablation. Source electrode 11 is grounded, a voltage of +0.01 V isapplied to drain electrode 12, and a positive voltage is applied to gateelectrode 13. A current flowing between source electrode and drainelectrode is observed, causing the switching element to transit to theon state. When a negative voltage is applied to gate electrode, thecurrent is reduced, causing the switching element to transit to the offstate. Then, after the switching element has transited to the off state,when a positive gate voltage is applied, the current flowing betweensource electrode and drain electrode is increased.

2nd Embodiment

The present embodiment is characterized in that the drain electrode inthe first embodiment is formed of the same material as that of the gateelectrode.

An arrangement of the switching element according to the presentembodiment will be described below.

FIG. 11 is a cross-sectional view of a structure of the switchingelement according to the present embodiment.

As shown in FIG. 11, switching element 50 has gate electrode 53 disposedon substrate 55 covered with an insulating film, ion conductor 54disposed on gate electrode 53, and source electrode 51 and drainelectrode 52 disposed on ion conductor 54. Source electrode 51, drainelectrode 52, and gate electrode 53 are electrically insulated from eachother.

Source electrode 51 and drain electrode 52 are disposed within oneplane. Gate electrode 53 and drain electrode 52 include a material forsupplying metal ions to ion conductor 54 based on an electrochemicalreaction. Each of source electrode 51 and ion conductor 54 is made ofthe same material as with the first embodiment, and will not bedescribed in detail below.

Operation of the two-electrode switching element based on sourceelectrode 51 and drain electrode 52 will be described below.

When source electrode 51 is grounded and a positive voltage is appliedto drain electrode 52, a metal filament grows due to copper precipitatedbetween source electrode 51 and drain electrode 52, electricallyinterconnecting source electrode 51 and drain electrode 52 to bringswitching element into the on state. After switching element has beenbrought into the on state, a negative voltage is applied to drainelectrode 52, dissolving the metal filament between source electrode 51and drain electrode 52 into ion conductor 54, disconnecting sourceelectrode 51 and drain electrode 52 to bring switching element into theoff state.

Operation of an arrangement similar to the above two-electrode switchingelement has been disclosed in the art (Applied Physics Letter, Vol. 82,No. 18, p. 3032 through 3034). The present embodiment resides in thatthe magnitude of the drain current is controlled by gate electrode 53.

Operation of the switching element of the above arrangement will bedescribed below.

After the voltage is applied to drain electrode 52 to interconnectsource electrode 51 and drain electrode 52 with the metal filament,thereby bringing the switching element into the on state, as describedabove, a positive voltage is applied to gate electrode 53, reducing theon-resistance and increasing the drain current. The reasons for thereduced on-resistance and the increased drain current are as follows:When the switching element is brought into the on state with sourceelectrode 51 and drain electrode 52 being interconnected, since theon-resistance is reduced, the voltage is less liable to be applied todrain voltage 52, thus failing to increase the precipitated amount ofcopper. However, when a positive voltage is applied to gate electrode53, it precipitates more copper between source electrode 51 and drainelectrode 52, thereby reducing the on-resistance.

After switching element has been brought into the on state, when anegative voltage is applied to gate electrode 53, the drain current isreduced, thus removing the metal filament in order to increase theon-resistance and further to bring switching element into the off state.

According to a specific example, if the on-resistance has a value of 10Ωand the drain voltage has a value of 0.1 V, then the drain current has avalue of 10 mA. The current value of 10 mA is very high in semiconductorintegrated circuits that have minute interconnect widths, tending toburn off interconnects unless they are thick enough and also to breakinterconnects due to the motion of atoms in the interconnects(electromigration). When a voltage is applied to gate electrode 53 whileswitching element is in the on state, it is possible to control theon-resistance to prevent an excessive drain current from flowing.

Immediately before a metal filament interconnects source electrode 51and drain electrode 52 when a predetermined positive voltage is appliedto drain current 52 for a predetermined period of time, a voltage isapplied to gate electrode 53 to bring switching element into the onstate. At this time, the voltage applied to gate electrode 53 may besmall, thus solving the problem of the two-electrode switching elementin which too large a drain current flows when the switching element isbrought into the on state. It is necessary to check in advance thepredetermined positive voltage that is applied to drain electrode 52 andthe predetermined period of time for which it is applied immediatelybefore a metal filament interconnects source electrode 51 and drainelectrode 52, and also to set the timing to apply the voltage to gateelectrode 53.

In the present embodiment, a voltage may be applied between sourceelectrode 51 and drain electrode 52 or a voltage may be applied to gateelectrode 53 in order to electrically interconnect source electrode 51and drain electrode 52.

A process of manufacturing the switching element according to thepresent embodiment will be described below. Those steps of themanufacturing process which are identical to those of the firstembodiment will not be described in detail below.

After gate electrode 53 and ion conductor 54 are formed as with thefirst embodiment, a titanium film is formed to a thickness of 10 nm bysputtering, and a gold film is formed to a thickness of 100 nm by vacuumevaporation. Then, a resist having a predetermined pattern is formed onthe gold film by lithography, and then the assembly is dry-etched toform source electrode 51 and the resist is removed. Thereafter, a copperfilm is formed to a thickness of 100 nm by vacuum evaporation. Then, aresist having a predetermined pattern is formed on the copper film bylithography. Thereafter, the area of the copper film which is notcovered with the resist is removed by ion milling, thus forming drainelectrode 52, and the resist is removed. The gap between sourceelectrode 51 and drain electrode 52 has a size of 100 nm or less.

While the titanium film is formed by sputtering in the above example, itmay be formed by vacuum evaporation. Lift-off may be used instead of dryetching to form source electrode 51 and drain electrode 52. As with thefirst embodiment, methods other than anodic polarization may be employedto form copper sulfide.

The switching element according to the present embodiment isillustrative only. Switching elements of other structures are possiblewithout departing from the scope of the invention. With the exceptionthat the drain electrode is made of the same material as the gateelectrode, the structure, layout, and the manufacturing method describedabove with respect to the first embodiment may be applied to the presentembodiment.

3rd Embodiment

An arrangement of an FPL circuit which incorporates switching elementsaccording to the present invention will be described below.

As described in the background art, the FPL circuit has a plurality oflogic circuit blocks, interconnects that interconnect the logic circuitblocks, and antifuse elements for changing the connection of theinterconnects. According to the present embodiment, switching elementsaccording to the present invention are used as programming elementsinstead of as antifuse elements.

FIG. 12 is a cross-sectional view showing a structure of a switchingelement according to the present invention as it is applied to an FPLcircuit.

The structure shown in FIG. 12 is similar to the first embodiment shownin FIG. 8A except that source electrode 21 in FIG. 8A is replaced withinterconnect A61 and drain electrode 22 in FIG. 8A is replaced withinterconnect B62.

Operation of the switching element shown in FIG. 12 will be describedbelow.

Interconnects A61, B62 are grounded and a positive voltage is applied togate electrode 63 or a negative voltage is applied to interconnects A61,B62 and gate electrode 63 is grounded, precipitating copper betweeninterconnects A61, B62 to electrically connect interconnects A61, B62 toeach other. Interconnects A61, B62 are grounded and a negative voltageis applied to gate electrode 63 or a positive voltage is applied tointerconnects A61, B62 and gate electrode 63 is grounded, dissolving theprecipitated copper to electrically disconnect interconnects A61, B62from each other.

The switching elements for use in an FPL circuit may be of the structureof the first embodiment shown in FIG. 8A or the structure of the secondembodiment.

A process of manufacturing the switching element shown in FIG. 12 willbe described below. Those steps of the manufacturing process which areidentical to those of the first and second embodiments will not bedescribed in detail below.

Insulating layer 65 is formed on a substrate on which logic circuitblocks and peripheral circuits have been formed. Then, insulating layer64 having an opening defined therein is formed on insulating layer 65,and gate electrode 63 and ion conductor 24 are successively formed inthe opening. Thereafter, interconnects A61, B62 are formed respectivelyin place of source electrode 21 and drain electrode 22 shown in FIG. 8A.

In an experiment, the FPL circuit which incorporates the switchingelements according to the present invention was capable of performing atleast several million rewriting cycles. Any signal delay caused by theFPL circuit is small because the on-resistance of the switching elementsis small. The FPL circuit is better than an FPL circuit employingconventional antifuse elements because it is rewritable, and is betterthan an FPL circuit employing EEPROMs because it causes a smaller signaldelay.

4th Embodiment

An arrangement of a memory device employing switching elements accordingto the present invention as information storage means will be describedbelow.

FIG. 13 is a circuit diagram of a memory device employing switchingelements according to the present invention.

As shown in FIG. 13, the memory device includes memory array 70 havingan array of memory cells, bit lines 73 a through 73 z, word lines 74 athrough 74 y, and word lines 75 a through 75 y. Memory cell 76, likeother memory cells, has cell-selecting MOS transistor 71 and switchingelement 72. Each of the bit lines and each of the word lines areconnected respectively to a decoder circuit and a driver circuit (notshown). Bit lines are shared by adjacent memory cells. Memory array 70and peripheral circuits (not shown) including the decoder circuits andthe driver circuits make up an integrated memory circuit.

In memory cell 76, MOS transistor has a source electrode connected tobit line 73 a and a gate electrode to word line 74 a. Switching element72 has a source electrode connected to bit line 73 b and a gateelectrode to word line 75 a. The drain electrode of switching element 72is connected to the drain electrode of MOS transistor 71.

Operation of the memory device thus constructed will be described below.Of stored information “1”, “0”, the stored information “1” isrepresented by the on state of a switching element, and the storedinformation “0” is represented by the off state of a switching element.A voltage required by a switching element to transit between the onstate and the off state, i.e., the difference between a gate voltage anda voltage applied to the source electrode, is represented by Vt, and anoperating voltage of MOS transistor 71 by VR.

For writing “1” in memory cell 76, the voltage Vt is applied to wordline 75 a that is connected to the gate electrode of switching element72 of memory cell 76, and a voltage of 0 V is applied to bit line 73 bthat is connected to the source electrode of switching element 72. Avoltage Vt/2 is applied to word lines 75 b through 75 y and bit lines 73a, 73 c through 73 z. As described above with respect to the firstembodiment and the second embodiment, switching element 72 is broughtinto the on state, writing the stored information “1” therein. At thistime, no stored information is written in the other switching elementsexcept for switching element 72, and these other switching elements holda state prior to the application of the voltage.

For writing “0” in memory cell 76, the voltage applied to word line 75 athat is connected to the gate electrode of switching element 72 ofmemory cell 76 is set to 0 V, and the voltage Vt is applied to bit line73 b that is connected to the source electrode of switching element 72.The voltage Vt/2 is applied to word lines 75 b through 75 y and bitlines 73 a, 73 c through 73 z. As described above with respect to thefirst embodiment and the second embodiment, switching element 72 isbrought into the off state, writing the stored information “0” therein.The other switching elements except for switching element 72 hold astate prior to the application of the voltage.

For reading the stored information from memory cell 76, the voltage VRis applied to word line 74 a to turn on MOS transistor 71, and thevoltage applied to the other word lines is set to 0 V, and theresistance between bit lines 73 a, 73 b is determined. This resistancerepresents the combination of the on-resistance of MOS transistor 71 andthe resistance of switching element 72. If this combined resistance istoo large to be measured, then switching element 72 can be judged asbeing in the off state, indicating that the stored information in memorycell 76 is “0”. If the combined resistance is smaller than apredetermined value, then switching element 72 can be judged as being inthe on state, indicating that the stored information in memory cell 76is “1”.

MOS transistor in each memory cell may be replaced with a diode.

The present invention is not limited to the above embodiments butvarious changes and modifications may be made and should be interpretedas falling within the scope of the present invention.

1. A switching element comprising: an ion conductor capable ofconducting metal ions therein; a first electrode and a second electrodewhich are disposed in contact with said ion conductor; and a thirdelectrode disposed in contact with said ion conductor and including saidmetal ions; wherein an interelectrode distance L1 between said firstelectrode and said second electrode, an interelectrode distance L2between said first electrode and said third electrode, and aninterelectrode distance L3 between said second electrode and said thirdelectrode satisfy the condition according to the expression:L1<L2×2 and L1<L3×2.
 2. The switching element according to claim 1,wherein the interelectrode distance between said first electrode andsaid second electrode is 0.5 μm or less.
 3. A switching element disposedon a substrate covered with an insulating film or an insulatingsubstrate, comprising: an ion conductor capable of conducting metal ionstherein; a first electrode and a second electrode which are disposed incontact with said ion conductor; and a third electrode disposed incontact with said ion conductor and including a metal dissolvable intosaid ion conductor based on an electrochemical reaction; wherein aninterelectrode distance L1 between said first electrode and said secondelectrode, an interelectrode distance L2 between said first electrodeand said third electrode, and an interelectrode distance L3 between saidsecond electrode and said third electrode satisfy the conditionaccording to the expression:L1<L2×2 and L1<L3×2.
 4. The switching element according to claim 3,wherein: said first electrode and said second electrode are disposed onsaid substrate in spaced-apart relation to each other, and theinterelectrode distance between said first electrode and said secondelectrode is 0.5 μm or less; said ion conductor is disposed to coversaid first electrode and said second electrode; and said third electrodeis disposed on said ion conductor.
 5. The switching element according toclaim 3, wherein: said third electrode is disposed on said substrate;said ion conductor is disposed on said third electrode; and said firstelectrode and said second electrode are disposed on said ion conductorin spaced-apart relation to each other, and the interelectrode distancebetween said first electrode and said second electrode is 0.5 μm orless.
 6. The switching element according to claim 3, wherein: said firstelectrode and said third electrode are disposed on said substrate; saidion conductor is disposed on said first electrode and said thirdelectrode; and said second electrode is disposed on said ion conductor,and the interelectrode distance between said first electrode and saidsecond electrode is equal to or on the order of a film thickness of saidion conductor.
 7. The switching element according to claim 1, wherein anelectrical characteristic between said first electrode and said secondelectrode is controlled by applying a voltage to said third electrode.8. The switching element according to claim 3, wherein an electricalcharacteristic between said first electrode and said second electrode iscontrolled by applying a voltage to said third electrode.
 9. Theswitching element according to claim 7, wherein said electricalcharacteristic represents electric conductivity.
 10. The switchingelement according to claim 8, wherein said electrical characteristicrepresents electric conductivity.
 11. The switching element according toclaim 1, wherein: said first electrode and said second electrode areelectrically interconnected to bring the switching element into an onstate by applying a voltage, which is positive with respect to at leastone of said first electrode and said second electrode, to said thirdelectrode; and said first electrode and said second electrode areinsulated from each other to bring the switching element into an offstate by applying a voltage, which is negative with respect to at leastone of said first electrode and said second electrode, to said thirdelectrode.
 12. The switching element according to claim 3, wherein: saidfirst electrode and said second electrode are electricallyinterconnected to bring the switching element into an on state byapplying a voltage, which is positive with respect to at least one ofsaid first electrode and said second electrode, to said third electrode;and said first electrode and said second electrode are insulated fromeach other to bring the switching element into an off state by applyinga voltage, which is negative with respect to at least one of said firstelectrode and said second electrode, to said third electrode.
 13. Theswitching element according to claim 1, wherein said second electrodeincludes a metal dissolvable into said ion conductor based on anelectrochemical reaction.
 14. The switching element according to claim3, wherein said second electrode includes a metal dissolvable into saidion conductor based on an electrochemical reaction.
 15. The switchingelement according to claim 13, wherein: said first electrode and saidsecond electrode are electrically interconnected to bring the switchingelement into an on state by either applying a voltage, which is positivewith respect to said first electrode, to said second electrode, orapplying a voltage, which is positive with respect to at least one ofsaid first electrode and said second electrode, to said third electrode;and said first electrode and said second electrode are insulated fromeach other to bring the switching element into an off state by eitherapplying a voltage, which is negative with respect to said firstelectrode, to said second electrode, or applying a voltage, which isnegative with respect to at least one of said first electrode and saidsecond electrode, to said third electrode.
 16. The switching elementaccording to claim 14, wherein: said first electrode and said secondelectrode are electrically interconnected to bring the switching elementinto an on state by either applying a voltage, which is positive withrespect to said first electrode, to said second electrode, or applying avoltage, which is positive with respect to at least one of said firstelectrode and said second electrode, to said third electrode; and saidfirst electrode and said second electrode are insulated from each otherto bring the switching element into an off state by either applying avoltage, which is negative with respect to said first electrode, to saidsecond electrode, or applying a voltage, which is negative with respectto at least one of said first electrode and said second electrode, tosaid third electrode.
 17. The switching element according to claim 1,wherein at least one of said first electrode, said second electrode, andsaid third electrode has a pointed portion on a surface thereof held incontact with said ion conductor.
 18. The switching element according toclaim 3, wherein at least one of said first electrode, said secondelectrode, and said third electrode has a pointed portion on a surfacethereof held in contact with said ion conductor.
 19. The switchingelement according to claim 1, wherein said ion conductor compriseseither a calcogenide material including an element belonging to thegroup 6B of the periodic table, or metal ionic glass, or metal ionicamorphous semiconductor.
 20. The switching element according to claim 3,wherein said ion conductor comprises either a calcogenide materialincluding an element belonging to the group 6B of the periodic table, ormetal ionic glass, or metal ionic amorphous semiconductor.
 21. Theswitching element according to claim 1, wherein: said ion conductor andsaid third electrode are made of either copper sulfide and copper,respectively, or of silver sulfide and silver, respectively; andportions of said first electrode and said second electrode which areheld in contact with said ion conductor are made of either a metal suchas platinum, aluminum, gold, titanium, tungsten, vanadium, niobium,tantalum, chromium, or molybdenum, a nitride of the metal, or a silicideof the metal, or a combination thereof.
 22. The switching elementaccording to claim 3, wherein: said ion conductor and said thirdelectrode are made of either copper sulfide and copper, respectively, orof silver sulfide and silver, respectively; and portions of said firstelectrode and said second electrode which are held in contact with saidion conductor are made of either a metal such as platinum, aluminum,gold, titanium, tungsten, vanadium, niobium, tantalum, chromium, ormolybdenum, a nitride of the metal, or a silicide of the metal, or acombination thereof.
 23. The switching element according to claim 13,wherein: said ion conductor and said third and second electrodes aremade of either copper sulfide and copper, respectively, or of silversulfide and silver, respectively; and a portion of said first electrodewhich is held in contact with said ion conductor is made of either ametal such as platinum, aluminum, gold, titanium, tungsten, vanadium,niobium, tantalum, chromium, or molybdenum, a nitride of the metal, or asilicide of the metal, or a combination thereof.
 24. The switchingelement according to claim 14, wherein: said ion conductor and saidthird and second electrodes are made of either copper sulfide andcopper, respectively, or of silver sulfide and silver, respectively; anda portion of said first electrode which is held in contact with said ionconductor is made of either a metal such as platinum, aluminum, gold,titanium, tungsten, vanadium, niobium, tantalum, chromium, ormolybdenum, a nitride of the metal, or a silicide of the metal, or acombination thereof.
 25. The switching element according to claim 1,wherein said ion conductor comprises an electrolytic solution.
 26. Theswitching element according to claim 3, wherein said ion conductorcomprises an electrolytic solution.
 27. The method of driving aswitching element according to claim 7, comprising the step of:controlling said electrical characteristic based on the voltage appliedto said third electrode and/or a period of time for which the voltage isapplied to said third electrode.
 28. The method of driving a switchingelement according to claim 8, comprising the step of: controlling saidelectrical characteristic based on the voltage applied to said thirdelectrode and/or a period of time for which the voltage is applied tosaid third electrode.
 29. The method of driving a switching elementaccording to claim 11, comprising the steps of: selectively bringingsaid switching element into said on state and said off state dependingon the polarity of the voltage applied to said third electrode; andholding said switching element in either said on state or said off stateeven if no voltage is applied to said third electrode.
 30. The method ofdriving a switching element according to claim 12, comprising the stepsof: selectively bringing said switching element into said on state andsaid off state depending on the polarity of the voltage applied to saidthird electrode; and holding said switching element in either said onstate or said off state even if no voltage is applied to said thirdelectrode.
 31. The method of driving a switching element according toclaim 15, comprising the steps of: selectively bringing said switchingelement into said on state and said off state depending on the polarityof the voltage applied to said third electrode; and holding saidswitching element in either said on state or said off state even if novoltage is applied to said third electrode.
 32. The method of driving aswitching element according to claim 16, comprising the steps of:selectively bringing said switching element into said on state and saidoff state depending on the polarity of the voltage applied to said thirdelectrode; and holding said switching element in either said on state orsaid off state even if no voltage is applied to said third electrode.33. The method of driving a switching element according to claim 11,comprising the steps of: when said switching element is caused totransit between said on state and said off state, measuring theconductivity between said first electrode and said second electrode; andcontrolling the voltage applied to said third electrode based on achange in the conductivity.
 34. The method of driving a switchingelement according to claim 12, comprising the steps of: when saidswitching element is caused to transit between said on state and saidoff state, measuring the conductivity between said first electrode andsaid second electrode; and controlling the voltage applied to said thirdelectrode based on a change in the conductivity.
 35. The method ofdriving a switching element according to claim 15, comprising the stepsof: when said switching element is caused to transit between said onstate and said off state, measuring the conductivity between said firstelectrode and said second electrode; and controlling the voltage appliedto said third electrode based on a change in the conductivity.
 36. Themethod of driving a switching element according to claim 16, comprisingthe steps of: when said switching element is caused to transit betweensaid on state and said off state, measuring the conductivity betweensaid first electrode and said second electrode; and controlling thevoltage applied to said third electrode based on a change in theconductivity.
 37. The rewritable logic integrated circuit incorporatinga switching element according to claim 7 as a programming switch. 38.The rewritable logic integrated circuit incorporating a switchingelement according to claim 8 as a programming switch.
 39. The memorydevice having memory cells each comprising a switching element accordingto claim 7 and either a MOS transistor or a diode.
 40. The memory devicehaving memory cells each comprising a switching element according toclaim 8 and either a MOS transistor or a diode.
 41. The memory deviceaccording to claim 39, wherein: said memory cell comprises a MOStransistor; said switching element has said second electrode connectedto the drain electrode of said MOS transistor, said first electrodeconnected to a first bit line, and said third electrode connected to afirst word line; and said MOS transistor has a source electrodeconnected to a second bit line different from said first bit line and agate electrode connected to a second word line different from said firstword line.
 42. A memory device according to claim 40, wherein: saidmemory cell comprises a MOS transistor; said switching element has saidsecond electrode connected to the drain electrode of said MOStransistor, said first electrode connected to a first bit line, and saidthird electrode connected to a first word line; and said MOS transistorhas a source electrode connected to a second bit line different fromsaid first bit line and a gate electrode connected to a second word linedifferent from said first word line.